A diagonal ventilating fan of this kind is known from the Assignee's DE 4 127 134 A1 and corresponding U.S. Pat. No. 5,695,318, HARMSEN. The fan wheel therein is in the shape of a truncated cone on whose conical surface the fan blades are arranged. It has in its interior a hollow-cylindrical portion, and pressed thereinto is the cup-shaped magnetic yoke of the external rotor of an electronically commutated (EC) motor. The internal stator of this EC motor is mounted on the outer side of a bearing tube, in which ball bearings for journaling the external rotor are located.
The disadvantage that results with this type of configuration of a diagonal ventilating fan is that the EC motor is poorly cooled, so that it can deliver only a low performance level, since it would be thermally overloaded at higher performance levels. It could be said that a diagonal ventilating fan is “cooling-impaired,” which correspondingly reduces its performance.
US 2007/0 205 676 A1, LAN et al., presents an axial ventilating fan in which the external rotor of the drive motor is mounted with a radial spacing in a fan wheel. The result is to produce around the external rotor an annular cavity through which cooling air flows during operation. The pumping effect for generating this cooling air stream is generated by centrifugal action when the ventilating fan is in operation.
In a ventilating fan of this kind, the flow direction of the cooling air can reverse when the dynamic pressure at the outlet rises, i.e. the cooling air then flows from the outlet to the inlet of the ventilating fan, since the pressure that is generated by centrifugal force is weak in relation to the pressure generated by the fan wheel. The reason is that when the ventilating fan is operating in a free-outlet manner, the cooling air is entrained, as it exits from the ventilating fan, by the main air stream generated by the latter.
If the ventilating fan is instead operating not in a free-outlet manner but instead against a dynamic pressure, the direction of the cooling air stream is then reversed.
During the time span during which the direction of the cooling air stream changes, the air in the ventilating fan is stagnant and its electric motor is consequently not cooled; this can result in rapid destruction thereof.